Electrical resistance element and furnace containing the same



April 22, 1958 w, c. sElFERT ELECTRICAL..RES1STANCE ELEMENT AND FURNACE''CONTAINING`- THE' SAME Filed Dec. 3. 1956 VACUUM PUMP www

INVENT OR WALL/,4M c. sE/FERT ATTORNEY ELECTRICAL RESISTANCE ELEMENT ANDFURNACE CONTAINING THE SAME William C. Seifert, Ardmore, Pa., assignorto Donald W. Kent, Philadelphia, Pa.

y' rApplication December 3, 1956, Serial No. 626,050

4 Claims. (Cl. 13-31) I This invention is a new and useful electricalresistance element of the direct radiation type and furnace containingthe same. The invention will be fully understood fromthe followingdescription read in conjunction with the drawing in which: v

Fig. 1 is a diagrammatic showing of one step in the manufacture of theelement of my invention;

Fig. 2 is a diagrammatic showing of a second step in the manufacture ofsaid element; y

Fig. 3 is a longitudinal section through a furnace containingv saidelectrical resistance element; p

Fig. 4 is a transverse section through the showing in Fig. 3 on theplane indicated by the line 4 4; and

Fig. 5 is a top view through part of the construction shown in Fig. 3 onthe plane indicated by 5-5. j

In my prior application, Serial No. 577,128, filed April 9, 1956, Idescribed an electrical resistance element of the direct radiation typeconsisting of an external, tubular sheath of high melting point metal,surrounding a core of high melting point inorganic oxide, and reduced indiameter uponsaid core by longitudinal extension to compress said coreto maximum density. Such resistance elements are produced in the form oflong, tubular rods, which arey then bent into loops at evenly spacedintermediate points to form aqseries of parallel rods of even length,

connected at their ends by congruent loops. The blanket heater so formedis held in the furnace by hanging the top.` loops on refractoryprojections set in the refractory inside walls of the furnace. Suchheaters are characterized by structural stability at high temperatures;they do not warp or distort even at temperatures in excess of 2000" F.They have uniform longitudinal resistance and circumferentially uniformradiation characteristics. Such heaters are preeminently adapted to usein furnaces operating under high vacuum although they may also be usedin furnaces in which special atmospheres are maintained.

When operating under vacuum the heat is of course communicated to thecharge in the heating zone exclusively by radiation, and in such case itis important that the heater elements be capable of transmitting thelargest possible amount of radiant heat in a given time. The instantinvention is an improvement on the construction described and claimed insaid prior application, by which the heat input is maximized and theradiation more uniformly and intensely distributed within the furnace.Although preeminently suited to use under vacuum, it is also'advantageous where the furnace is operated with air or a specialatmosphere.

4Referring to Fig. 1, tube 1 is made of a metal which is cold-workableand which has a high residual, tensile strength and an extremely lowvapor pressure even at temperatures in excess of 2000 F. The exactcomposition will be determined by whether it is to be installed in afurnace operated under vacuum or in which a special atmosphere is to bemaintained. For furnaces operating under'high vacuum, the following areexamples of alloys which are suitable:

Example I Percent Nickel 77-80 Chromium 17-20 Aluminum l- 3 Smallamounts of manganese, carbon, etc.

Example 2 Percent Iron 81-85 Chromium 12-15 Nickel 3 4 In general thealloys conventionally known as NichrOme, Chromel and Inconel aresuitable. The alloys commonly known as the stainless irons are alsosuitable. For use at temperatures considerably in excess of 2000 F.platinum may be employed.

For the purpose of fabricating the element 1 of my invention, the lowerend of tube 2 of the specified composition is sealed by squeezed portion3 and the upper end is connected to hopper 4 provided with vibrator 5and containing finely divided, high-melting point, inorganic oxide 6,suc-h as alumina, zirconia, or magnesia. By operating vibrator 5, tube 2is completely filled with oxide. For certain critical uses thisoperation will be performed under vacuum. The other end of tube 2 isthen sealed by squeezed portion 7 and engaged by gripper jaws 8 uponwhich traction is exerted by chain 9 to draw the tube through swagingdie 10 secured in holder 11. The drawing reduces the external diameterwithout substantially affecting the wall thickness, thereby reducing theenclosed volume and compressing the contained oxide. The reduction iscarried to the point at which the oxide has reached maximum density, i.e., at which the voids are substantially eliminated. This results in acurrent carrier characterized by extreme uniformity of thermal radiationcombined with exceptional strength and rigidity at extremely hightemperatures and substantially complete freedom from warping.

Referring to Figs. 3 and 4, the furna-ce 12 is enclosed in a shellconsisting of tubular section 13 terminating in bumpedl ends 14 and 15.Bumpedend 14 is connected through pipe 17 to vacuum pump 18 dischargingthrough pipe 19. Bumped end 15 carries the anged opening 21 which may besealed, vacuum-tight, by dished cover 22 suspended from the crane arm23. Internally shell 13 is provided with refractory lining 24, definingside walls Z5 and 26, roof 27 and lioor 28. If a special atmosphere isto be maintained in the furnace, the gas suitable for this purpose maybe introduced through the valved pipe 29 (Fig. 3).

By reference to Figs. 4 and 5 it will be noted that each of the sidewalls carries a series of refractory projections 30 which extendhorizontally at a right angle to the wall, and thence at an angle ofabout 45 to form supports for the resistance element 31.

In preparing the resistance element 31 the tube 2 hereinabove referredto, kswaged upon the refractory filling and elongated by swaging, isbent into loops at evenly spaced intervals to form a series of parallelrods, spaced apart between 3 and 6 diameters on centers, which can besuspended from these projections or supports without touching the floor28 of the furnace. The tubes as produced by swaging are long enough sothat the free ends may be bent at a right angle, as indicated bynumerals 32 and 33 in Fig. 4, to be carried through the shell 13 CePatented Apr. 22, 1958 both pairs, is bent with reference to the nextsucceeding pair in .such a way that the median plane of each intersectsat an included angle gb of between 75 and 105 and preferably ofsubstantially 90. This construction is evident from the top viewthereofin Fig. 5. Before the bumped end 15 is placed in position and sealed toshell 13, the heater 31 soforrnedis carried into the space defined bykthe refractory lining 24. The` upper ends of the loops are hung on thefree ends of the projections 30 extending from one of the side walls 25or 26, with .the free ends of the heater extending through therefractory lining and through stuffing boxes such as 34, to permit ofthe establishment of the necessary electrical connections to include theheater 31 in an electric heating circuit. The free ends which projectthrough the refrac- :tory lining and the stufling boxes are reinforcedwith cxternal metal .sleeves to lower the resistance at this point.Temperatures obtaining in the furance are measurable by thermo-couple 36in combination with any suitable recording instrument (not shown).

Referring to Fig. 4, transformer 41 receives current lthrough primaryconnections 42 and 43 andsupplies current through secondary posts 44 and45. The end 32 of heater 31 shown in the figure is electricallyconnected through lug 46, conductor 47'and lug 48, to secondary post 44.The end 33 of the tubular heater carried by. the wall 26 .shown in theiigure is electrically connected through lug 51, conducto-r 52 and lug53, lto secondary post 44. The other ends of the tubular heaters aresimilarly connected with post 45 through similar conductors and lugs.

In operation the tubular heaters may, for example, be maintained at atemperature of about 2150 F. tohold the furnace interior within 50 F. ofthe resistor. tem. perature. Radiation within the interior of thefurnace is extremely intense and of uniform distribution. If the furnaceis to be operated under vacuum, the Valve in valved pipe 29 remainsclosed, and aftercharging and before heating, the desired degree ofvacuum is lcreated within the furnace by the operation of vacuum pump18. If the furnace is t be operated with a special atmosphere followingevacuation, the gas necessary for this purpose isintroduced throughvalved pipe 29.

I claim:

1. An electrical resistance element of the direct radiation typeconsisting of an external tubular sheath of high melting point metalsurrounding a core of' high melting point inorganic oxide and reduced indiameter upon said core by longitudinal swaging .to compress said coreto maximum density, said tube bent to form a continuous series ofparallel vertical rods connected at their ends by congruent loops, eachconnected pair of rods being spaced apart between 3 and 6 diameters oncenters and the median plane of each vsucceeding pair sharing one rod incommon intersecting at an included angle between and 105.

2. An electrical resistance heater according tokclaim 1 in which saidmedian planes intersectat an angle of about 3. A resistance typeelectric furnace comprisingy a refractory setting defining` a heatingzone, and within said heating zone an electrical resistance element ofthe direct radiation type comprising an external tubular sheath of highmelting point metal surrounding a core of high melting point inorganicoxide reducedin diameter by longitudinal swaging to compress said coreto maximum density, said tubular sheath beingbent ytoform a continuousseries of parallel rods wit-h.connecting loops at their ends, eachconnected pair of rods being lspaced apart between 3 and 6 diameters oncenters, the median-plane of each succeeding pair sharing one rod incommon intersecting at an included angle between 75 and 105, saidelectrical resistance Velement positioned in saidfumacel with said rodsvertically oriented and with the loops at the upper ends of said rodscarried by projections extending horizontally from a side wall of saidfurnace.

4. A resistance type electric furnace according to claim 3 including, inaddition, a vacuum-tight shell enclosing said heating zone and means forevacuating said shell` and said heating zone.

References Cited in the file of this patent UNITED STATES PATENTS953,774 Appelberg Apr. 5, 1910 1,359,400 Lightfoot Nov. 6, 19201,555,542 Young Sept. 29, 1925 1,849,288l Fahrenwald Mar. 5, 19322,717,915 Shapiro Sept. 13, 1955 FOREIGN PATENTS 11,128 Great Britainvof 1908 142,876 Switzerland Dec. 16, 1930

